Replacing a Michelin LTX M/S tire on a 2007 Tacoma

Last night, I was leaving a friend's house where a short stone wall was being built in his front lawn that extended into the actual driveway. I didn't see the wall in the dark and the front passenger-side tire was punctured when the tire hit the corner of the short wall.

It's a sidewall puncture, so I'm sure that I'll have to replace tire, as opposed to repair. I bought all four of my Michelin LTX M/S tires on June 11, 2009 and they're still in great condition, with slightly less than 20,000 miles on them.

Since these tires are just over a year old, would it be best to buy two new tires, instead of simply replacing one tire? (Then, probably placing the two new tires on the back of my 4x2 regular cab truck and using the two existing rear tires in the front, perhaps.)

I'd hate to buy two or four new tires, since the Michelin LTS M/S tires that I have are only just over a year old, but I'm prepared to do this if need be.

Has anyone replaced just one single tire on their Tacoma without replacing two in a pair? Any suggestions, if you have done so?

Thanks.